1. Field of the Invention
The subject invention generally pertains to refrigeration systems and more specifically to auxiliary refrigeration systems that use outside air for the cooling medium.
2. Description of the Related Art
Conventional refrigeration systems for walk-in coolers and other refrigerated enclosures almost always utilize a compressor, a condenser and an evaporator in order to remove heat from the space to be cooled. Such conventional systems are reliable and effective at performing this function, though the electrical energy consumed by such systems is substantial. One method of reducing the electricity needed to refrigerate an enclosure is to use an outside air refrigeration system that utilizes the cooling potential of cold outside atmospheric air whenever that air becomes cold enough to cool the enclosure more efficiently than can the conventional refrigeration system. Because cooling with outside air typically involves simply moving the air with fans, it is inherently more energy efficient than a more complicated conventional refrigeration system, if the outside air temperature is sufficiently cold, sometimes as little as 4 degrees (F) cooler than the temperature of the air inside the enclosure. The colder the outside air temperature gets the more energy efficient an outside air refrigeration system becomes, and the colder the climate the more energy and money that can be saved by utilizing an outside air refrigeration system. When the outside air temperature is 30 degrees F. cooler than the air inside the enclosure an outside air refrigeration system can be as much as ten times as efficient as a conventional refrigeration system. In roughly the northern half of the United States the outside temperature is low enough for a great enough time during the year to justify the installation of an outside air refrigeration system. Since a typical refrigeration temperature for perishable food is between 33 and 40 degrees F., there are, of course, few places where the outside atmospheric air temperature does not at times warm up to a point where outside air cannot be used for refrigeration, so to maintain constant, reliable refrigeration an outside air refrigeration system must usually be used in conjunction with and auxiliary to a conventional refrigeration system.
There have been a number of auxiliary outside air refrigeration systems proposed. Some of these systems, such as those described in U.S. Pat. No. 4,175,401 and 4,023,947, employ a control system having a "changeover" thermostat that senses the outside temperature and de-energizes the conventional refrigeration system and energizes an outside air refrigeration system whenever the outside temperature falls below a pre-determined temperature, typically a temperature that will usually be cool enough to refrigerate the enclosure regardless of the cooling load. Only one or the other of the two systems can operate at any one time, but not both. A problem with having a pre-selected "changeover" temperature setting is that the setting may at times be too warm, such that the cooling load of the enclosure is too great for the cooling capacity of the outside air system and the temperature inside the enclosure can rise to an unacceptable level. This can occur when a large warm load of product is introduced into the enclosure or doors to the heated portion of the building are opened frequently or for long periods and admit warm air into the enclosure. At other times this same changeover temperature setting may be too low. This can occur when the cooling load of the enclosure may be so low that outside air only a few degrees cooler than the air inside of the enclosure could satisfactorily refrigerate the enclosure, but is prevented from doing so because the low changeover temperature setting will not allow the thermostat to energize the outside air fan or fans. This results in a lost opportunity to save energy as the less energy efficient conventional refrigeration system will operate more than it needs to.
Another control strategy for outside air systems is to have no electrical interconnection between the conventional refrigeration system and the outside air system. This type of "independent" system is found in U.S. Pat. Nos. 4,250,716, 4,178,770, 4,147,038, 4,619,114, 4,244,193, and 4,358,934. The operation of each of these outside air systems is controlled by two thermostats, one sensing the outside temperature and one sensing the temperature inside the enclosure. The thermostat controlling the opeartion of the conventional refrigeration system is set at a higher operating range than the thermostat sensing the enclosure temperature for the outside air system. The conventional refrigeration system does not operate as long as the outside air system can adequately cool the enclosure. The outside air thermostat is set at a pre-determined cut-in temperature such that the outside air system will only be used when the outside air is cold enough to always be at least as efficient as the conventional refrigeration system. An "independent" system is preferable to a "changeover" type system because it allows simultaneous operation of both the conventional refrigeration system and the outside air system. The cut-in temperature setting of the outside air thermostat can be such that the outside air used is just cold enough to contribute to the refrigeration of the enclosure, and does not have to be cold enough to handle the refrigeration load alone, without help from the conventional refrigeration system. This results in the more efficient outside air system handling more of the refrigeration load in an "independent" system than it would with a "changeover" system and therefore more energy and money saved. However, a given cut-in setting of the outside thermostat of an "independent" outside air system can at times still be too low to make full use of the cooling potential of outside air. When the cooling load of the enclosure is great and the conventional refrigeration system cannot keep the temperature of the enclosure from rising, the pre-determined cut in temperature setting of the outside air thermostat may prevent the outside air system from operating, even though it could, given the temperature differential between the inside and outside air, more efficiently refrigerate the enclosure than can the conventional refrigeration system. This represents a lost opportunity to save energy. Raising the cut-in setting of the outside air thermostat too high can cause wasted energy when the temperature differential between the inside and outside air is small and the conventional refrigeration system is more efficient than the outside air system.
It can be seen that a given outside air refrigeration system can be more efficient than the conventional refrigeration system it is auxiliary to, but only when the temperature differential between the outside air brought in and the enclosure air is great enough. Though it can vary greatly depending on the characteristics of the specific installation, this differential is typically about 4 degrees F. In this typical installation it is desirable to allow the outside air system to operate when the differential is 4 degrees or greater but not when the differential is only 3 degrees F. Because the temperature of the enclosure changes constantly, the temperature of the outside air at which it is desirable to operate the outside air system also changes constantly. A control system that does not respond to these changing conditions cannot maximize the energy savings while maintaining reliable refrigeration.
With neither the "changeover" nor the "independent" type of system is outside air automatically available to supplement the conventional refrigeration system whenever the outside air temperature is above the changeover or cut-in setting of the outside air thermostat, even when the cooling capacity of the outside air is adequate for the cooling load, or when the conventional refrigeration system is broken down or not functioning properly. In the case of a breakdown of the conventional system the enclosure temperature might rise all the way to the temperature of the surrounding heated building even if the outside temperature is many degrees cooler than that. In other words, if the changeover or cut-in setting of the outside air thermostat is 32 degrees F. then 33 degree F. outside air is not available to cool the enclosure even if the enclosure temperature rises to 40, 50, 60, or even 70 degrees F.
One common problem with outside air refrigeration systems (U.S. Pat. Nos. 4,250,716; 4,175,401; 4,023,947; 4,676,073; and 4,244,193) is that they allow pressurization of the enclosure because the pressure of the air being forced into the enclosure is not balanced by negative pressure from air being exhausted from the enclosure by another fan. Such pressurization results in cool air being forced out of the enclosure wherever it can escape, not just through the openings provided to the outside. Some of that cool air flows into the heated portions of the building, through open walk-in and reach-in doors and around imperfect gaskets for those same doors when they are closed. This results in increased energy use to heat the air to the higher temperature of the heated portion of the building.
In most conventional refrigeration systems the evaporator fans operate continuously. Their main purpose is to force air over the evaporator coils in order to transfer heat to the refrigerant inside the coils. After the cooling thermostat has been satisfied and the compressor and condenser fan have been de-energized the air forced through the evaporator by the fans continues to lose heat until the evaporator and any refrigerant are no longer colder than the rest of the enclosure, typically several minutes after the compressor has shut off. This period of time in which the evaporator fans operate after the compressor has shut off serves a useful purpose in that it helps to melt any condensate frost which may have built up on the evaporator coils during the time of compressor operation. Another purpose of the evaporator fans is to circulate the air within the enclosure so that the temperature is substantially the same throughout. Once the the residual coldness and condensate frost build-up been removed, this circulation is the only reason to want the evaporator fans to continue to operate. The electrical energy needed to operate the fans is substantial, and since all that electrical energy is converted to heat which adds to the cooling load and must be removed from the enclosure through increase operation of the refrigeration equipment, the energy cost of running the evaporator fans is compounded. It has been estimated that the average refrigeration compressor must operate two hours just to remove the heat generated by the evaporator fans in one day. The evaporator fans used are commonly selected based on their ability to transfer heat to the evaporator coils. A fan large and powerful enough to effectively remove the necessary heat from a enclosure is about ten times as large as it needs to be to simply circulate the air to even out the the temperature within the storage room. An outside air refrigeration system results in the compressor and condenser fan of the conventional refrigeration system being idle for days, weeks, or even months at a time. Evaporator fan operation is therefore only useful as a grossly overpowered circulating fan for much of the year. What is needed is a control that turns off the evaporator fans when they are not needed for evaporator cooling and defrosting and that energizes a much smaller circulating fan when the evaporator fans are not operating. Energy would be saved not only when the outside air system operates but anytime during the year the compressor is not operating. None of the outside air refrigeration systems mentioned accomplish these goals.